Disc optical-recording media such as CD and DVD media are widely used among optical-recording media, and there has been an increasing demand for the higher-density and higher-capacity in such optical-recording media with advancement of information technologies, e.g. development of multimedia or network.
Downsizing the area in which a specific amount of data is recorded, in other words, reducing the size of recording mark is one method for achieving high-density. In order to reduce the size of recording marks, it is necessary to make the spot size of light smaller by narrowing the light beam. Spot size of light is proportional to the amount of λ/NA, where λ is a wavelength of light and NA is a numerical aperture of lens. Therefore, in order to reduce the spot size of light, either making smaller wavelength of light or making larger a numeric aperture of lens is applied. Recently, the DVD technologies in which the light having a wavelength of 650 nm is applied have started to be practically utilized, and further the technologies aiming for high-density using the light having a wavelength of about 400 nm are being actively developed.
In order to realize high-density, it is necessary to increase the liner-recording density and track density by narrowing the interval between the marks while reducing the size of recording marks. When achieving high-density by making track pitches smaller, certain problem arises, i.e., crosstalk between adjacent tracks. Crosstalk is such a phenomenon that signals recorded on adjacent tracks during the reproducing. In addition, during recording or erasing the signals, other problem arises, that is, cross-write in which signals are recorded wrongly on adjacent tracks, and cross-erase in which recording marks are wrongly erased from adjacent tracks.
When using a pick-up having a wavelength of about 405 nm and a numerical aperture NA of 0.85, which is being put into practice, the spot size is 0.82×405/0.85=about 390 nm. Herein, the figure 0.82 is a constant which is generally used when the profile of light is in accordance with a Gaussian distribution. Since the spot size is in accordance with the Gaussian distribution, the constant is defined as 1/(e2) value of the peak of the distribution. There is a limit in collecting light. Therefore, when achieving high-density further by narrowing track pitch, the optical-recording media are required to be improved more.
When attempting to achieve high-density by making the length of recording marks shorter, a problem arises that the displacement of recording marks affects more adversely the error rate. In other words, the displacement of recording marks becomes relatively large, causing a remarkable increase in jitter of signals.
In order to solve such problems, a method which utilizes the difference of thermal conductivity is developed. For example, Patent Literature 1 discloses a method for reducing the cross-write and cross-erase is disclosed, wherein heat conduction is prevented by disposing layers having different thermal conductivities, thereby cross-write and cross-erase are reduced. In this method, the recording is conducted by absorbing light mainly in the recording layer and generating heat.
Patent Literatures 2 to 7 disclose a method for suppressing the generation of such phenomenon as cross-write and cross-erase while the track pitch is narrow as follows. Specifically, in the method, a recording area formed on a disk substrate is divided by track, and a region of a material having a thermal conductivity smaller than that of recording material constituting the recording area is interposed. This structure makes it difficult that the temperature rise in the beam spot, caused by light beam irradiation during the recording and erasing of signals, passes to the adjacent tracks. These Patent Literatures also disclose a method for diminishing the jitter during the reproducing of signals by utilizing a structure in which the recording area is divided not only between tracks but also within a track, i.e., divided by recording mark. In this case, too, the recording is conducted by absorbing light in the recording layer and generating heat.
In the case where signals are recorded by utilizing the heat generated from the absorption of light in the recording layer, the recording layer absorbs light, the temperature rises to the degree at which the recording is possible, and then signals are recorded. Thus, narrower track pitch causes the adjacent recording area to be within the light spot even after dividing the recording layer. As a result, recording marks are formed even on the recording layer in the recording area of adjacent tracks, which makes it difficult to prevent the cross-write and cross-erase perfectly. In addition, this method requires use of photolithography and the like, which makes manufacturing process complicated and thus production cost becomes expensive. Further, there is other problem that the method using photolithography is not suitable for preparing an optical recording medium with large area.
Patent Literature 8 discloses an invention in which the thickness of a recording layer on the side wall of the groove between the groove and land is thin, thereby reducing cross-erase.
In Patent Literature 9, conductors in a line form are provided and signals are recorded on the space between the conductors. The conductor allows the heat to be dissipated well and recording may be conducted without spreading of the recording marks. Specifically, edge of laser beam is intercepted and the heat is dissipated by the conductor, thus reducing influence on the adjacent tracks.
Patent Literature 10 discloses an information-recording medium employing a method to limit the area on which the recording marks are formed. In the method, metal layers in a line form are provided periodically, thereby periodical change of thermal conductivity is allowed and the area is limited. In these Patent Literatures, cross-write and the like are prevented by providing a metal film in a line manner as a base and utilizing the difference of the heat conductivities.
Patent Literatures 11 and 12 disclose optical-recording media using light-intercepting structures. These Literatures intend to reduce cross-talk, and the effect on the prevention of the cross-write and/or cross-erase is not apparent. Particularly, Patent Literature 11 discloses an invention in which the sum of the width of the track pitch and light-intercepting film is adjusted to the spot size of laser beam. However, in the case where, for achieving further high-density, the track pitch becomes small relative to the light spot and light spot extends to adjacent tracks, a new problem arises.
Patent Literature 12 discloses an invention which suppresses the cross-talk utilizing a mask effect based on the optical phase difference or utilizing a mask effect based on the difference in the optical properties caused by the difference of crystalline phase. In this invention, too, the effect on the prevention of the cross-write and/or cross-erase is not apparent. Further, when the masks are used for optical-recording media in which heat is utilized for recording, wide marks are obtained due to the influence of heat conductance. Thus, it is difficult to prevent the spreading of the recording marks over the adjacent tracks. Besides, the recording marks are recorded even on the masked portions since mask layer itself generates heat. Therefore, the cross-write may not be suppressed effectively.
Patent Literature 13 discloses an optical-recording medium in which a projected deformed portion is provided in the boundary between adjacent recording areas and the distance for the heat to travel is made to larger, thereby reducing cross-write and cross-erase in the adjacent portions. Even if the distance for the heat to travel is made to larger, when recording density is increased, the heat may spread and influence adjacent tracks because the layers constituting the optical-recording medium are formed in a continuous manner. In addition, this optical-recording medium is restricted to groove recording alone.
Patent Literature 14 discloses an invention in which the cross-erase is prevented by deepening the depth of the groove, or when the groove is shallow, the thickness in the vicinity of the boundary between the groove and land is reduced or a film is removed.    Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 2004-158091    Patent Literature 2 Japanese Patent Application Laid-Open (JP-A) No. 2000-276770    Patent Literature 3 Japanese Patent Application Laid-Open (JP-A) No. 2001-236689    Patent Literature 4 Japanese Patent Application Laid-Open (JP-A) No. 2000-251321    Patent Literature 5 Japanese Patent Application Laid-Open (JP-A) No. 2003-263805    Patent Literature 6 Japanese Patent Application Laid-Open (JP-A) No. 2001-266405    Patent Literature 7 Japanese Patent Application Laid-Open (JP-A) No. 11-176021    Patent Literature 8 Japanese Patent Application Laid-Open (JP-A) No. 11-53763    Patent Literature 9 Japanese Patent Application Laid-Open (JP-A) No. 2003-228880    Patent Literature 10 Japanese Patent Application Laid-Open (JP-A) No. 2003-217176    Patent Literature 11 Japanese Patent Application Laid-Open (JP-A) No. 03-290842    Patent Literature 12 Japanese Patent Application Laid-Open (JP-A) No. 08-63782    Patent Literature 13 Japanese Patent Application Laid-Open (JP-A) No. 2003-228885    Patent Literature 14 Japanese Patent Application Laid-Open (JP-A) No. 08-124211